1. Field of the Invention
The present invention relates to a purification device for charged particle beam diaphragm, and in particular to a purification device for charged particle beam diaphragm suitable for use in a charged particle beam device such as an electron microscope, an X-ray microanalyzer with an electronic probe, an ion microanalyzer and an electron beam drawing device for producing semiconductors.
2. Description of the Prior Art
The prior art charged particle beam devices as referred to above are customarily provided with a charged particle beam diaphragm for limiting the angular aperture of the charged particle beam. For example, an electron beam diaphragm of an electron microscope of scanning type referred to as an object diaphragm comprises a thin plate of molybdenum having a thickness of approximately 10 .mu.m. The thin plate is provided with a plurality of apertures which are close to 100 .mu.m in diameter.
In general, the diaphragm is baked out in vacuum, and the contaminant such as an oxide on the surface of the diaphragm is then cleared prior to its use. If the surface of the diaphragm is subject to the electron beam, however, hydrocarbon in vacuum is deposited on a part of the diaphragm surface whereto the electron beam is applied. Since the contamination thus deposited is nonconductive, a charged builds up and hence the electron beam which is the charged particle beam is deflected improperly.
In one method for solving the above described problem, the thickness of the diaphragm plate is made so thin that the heat generated by the application of the charged particles may reduce the deposition that is the cause of contamination. In another method for solving the above described problem, the diaphragm plate is used while it is being heated by a current flowing through it.
Contamination is caused not only on the diaphragm plate but also on a holder for holding the diaphragm plate. However, the contamination caused on the holder cannot be substantially removed by means of the above described methods. In general, therefore, the diaphragm plate holder is periodically removed from the vacuum chamber and baked to be purified.